Sometimes when I queue up with the dungeon finder, my random dungeon queue pops, I ready up and then the group forms, but instead of In Progress, it says Activity Complete.
The result is that everyone has to travel to the dungeon or player there to start. When you complete the dungeon you get no credit for the random daily dungeon reward.
This may be happening when a group is already formed and requeues for another dungeon to get a replacement.
Another recent but possibly related issue is that when I'm in a dungeon and requeue to find a replacement, I'm unable to leave queue and requeue. I would leave queue before to change my class roles, or toggle the finder when it seems stuck and unable to find anyone - this often worked.